Updated MeSH Model. Fixed descriptors and major topic.

[EssentiallyNoah]

MeSH descriptors can be up to 10 characters long.
REF: https://www.nlm.nih.gov/pubs/techbull/ma13/ma13_mesh_ui_expand.html

MeSH model is_main_topic should be is_major_topic. REF: https://docs.openalex.org/about-the-data/work#mesh

[dpriskorn]

Thanks! LGTM 😀